Do you know the weights on the TT and payload and towing capacity of the Expedition? Ever been to a scale? Have you added air to the TT tires since the last season? What psi do you run on the Expy when towing and does it have P or LT tires? What spring bar rating do you have?
Internet specs say the TT is 32 1/2' long, 7800 GVWR with dry hitch weight of 850 lbs. I would expect the tongue wt. to be around 1,000 - 1,100 lbs with the trailer fully loaded for camping. Internet says the Expedition El can tow max. 8900 lbs and max. receiver load is 890 lbs. Payload capacity looks to be 1620 lbs but depends on options. Not sure on the GCWR.
It looks like you will be well over the max. receiver load (tongue wt.). With just one passenger and groceries + camping gear in the Expy, payload will be around 400-500 lbs. Add one or more kids and dog, the payload in the Expy could be closer to 1,000 lbs. It looks you will loaded near or even well over the payload capacity.
So, a first look at the numbers suggests that you will be over the receiver tongue weight rating and close to or over on the Expy payload capacity. Don't know about the GCWR or RAWR. Basically you are pushing or exceeding the Expy's capabilities.
I would say the extra wt. in the holding tanks(s) would have little or no affect on handling/sway. If the tire pressures are down, that will definitely affect handling/sway. (Please note that it is extremely important to have the correct psi in the trailer's tires because if you don't, the resulting internal heat will damage the tires, increasing the risk of a blowout.) Under-rated spring bars could contribute to poorer handling/sway. When the TT is hooked up, is it level to slightly nose down?
I would go to a scale asap and calculate your max. payload capacity, tongue weight and TT weight. Then you will know exactly where you stand on actual weights and can make an informed decision from there. A photo with the TT hooked up and a closeup of the WDH might help.